I recently got interested in astronomy and star gazing in my free time and found that there were all kinds of interesting things that can be used to make astronomy more interesting. One of the first things I found I wanted was a red flashlight to help me read star charts, read settings on my telescope, select eye pieces, locate things that fell in the grass, walk around without bumping into things, and much more!
Why do astronomers use a red flashlight? There are a lot of good explanations for this on the web. The short answer is that astronomy is much easier to do when your eyes have adapted to the dark for 15 minutes or more. This adaptation can be reversed instantly when one is exposed to light - especially white or blue light. So, a dim red light can be incredibly handy when doing astronomy. So much so, that nearly all astronomers carry them when they're out star-gazing, astro-imaging, or conducting any low light activity.
So, I'm kind of a frequent maker. Whenever I have the need for something new - a new tool or toy for instance - I try to make something I have work, or create something new before buying something. This led me on an interesting quest to design a red flashlight for astronomy. That quest led me all over the place looking at PWM circuits and Arduino control for pulsing LEDs to control brightness. I got so excited that I spent a bit of time designing an adjustable red LED flashlight using a 555 timer circuit.
Alas, my cheapness took over when I built a throwie (see tons of instructables on this). The throwie includes a 2032 (or similar) lithium battery, an LED, and some tape. That's it. I used a red LED throwie for astronomy for a while, but found it somewhat inconvenient to tape and untape. I wanted a switch, and a case that I could set down and put into a bag. Along came a star party in PA (The Cherry Springs Star Party) in 2017 and I found that I needed a minimum of 5 low price, functional red flashlights for my family to use over the course of a one night stay at a star party. The throwie came to mind, but would have been inconvenient for my wife and kids. So, I decided to modify some cheap tea lamps that we bought on Amazon. This instructable will show you how to make one or several low-cost, functional, red LED lamps for astronomy using parts from Amazon and a soldering iron.
Step 1: Astronomer's Tea Lamp - Parts List and Cost
This project is made from a pretty simple parts list as shown below.
1. Tea Lamps (Amazon $13.99/24 lamps)
2. 50 Ohm Resistors (Amazon $7.99/25 50 Ohms)
3. Red LEDs (Amazon $7.95/10 reds)
I use these parts for other things, so cheaper alternatives are available. I bought 24 tea lamps because we use them for a lot of things around the house. I bought a set of mixed resistors because I use them for other projects. I bought a mixed set of LEDs because I wanted other colors. You could buy a smaller lot of tea lamps, only 50 ohm resistors, and only red LEDs.
Using my parts list above the cost would be $13.99+$7.99+$7.95=$29.93 for one to ten. $3 each is not too bad. It's cheaper than a dedicated red LED flashlight, but it could be a lot cheaper. Since I plan to use all the parts from the kits I only spent 13.99/24+7.99/750+7.95/100=$0.67/lamp! Not bad!
Step 2: Astronomer's Tea Lamp - Build Overview
The build for this project is very easy.
1. Disassemble a tea lamp.
2. De-solder the existing white LED.
3. Solder the 50 Ohm resistor to the red LED (in the correct orientation).
4. Solder the 50 Ohm resistor and red LED to the tea lamp.
5. Test it!
6. Reassemble the tea lamp.
7. You're done!
Step 3: Disassemble a Tea Lamp
Remove inside of tea lamp using needle nose pliers as shown.
Step 4: De-solder the Existing White LED.
Make a note of the orientation of the existing LED before removing it. LEDs are polarized, so the electricity will only flow in one direction. The red LED must be oriented the same as the one you're removing. If you look inside the bulb you can see that the positive and negative sides are different. You will use the features inside the bulb to correctly orient the red LED when you get to the next step.
De-solder the existing white LED and remove any residual solder from the switch contact.
Step 5: Solder the 50 Ohm Resistor to the Red LED (in the Correct Orientation).
I like to bend the resistor into the shape shown in the picture and solder the parts together in a third hand device. Make sure to use flux to make a nice joint. If you want to test now you can connect the LED lead and resistor lead to the battery to make sure you soldered to the correct leg. Cut off the excess LED lead and resistor wire.
Step 6: Solder the 50 Ohm Resistor and Red LED to the Tea Lamp.
Feed the LED through the slotted hole in the plastic and bend it into a "V" shape in the battery compartment. Solder the resistor to the switch where the white LED used to connect.
Step 7: Test It!
Put the battery back in (in the correct orientation). Turn on the switch. Does it work? If so you're good to go! If not, you might have to rebuild with the LED leads reversed. This will cost you an LED, but there's no way to rework it now.
Step 8: Reassemble the Tea Lamp.
Make sure to orient the lamp correctly and push the parts back together. Install the battery and battery cover if you haven't already. You're done! Congrats. Now you can light the way in the dark without affecting your low light visual adaptation.